For the purpose of increasing the directivities of signals of a MIMO communication system, the one dimensional precoder codebook has been upgraded into a two dimensional precoder codebook. An Evolved NodeB (eNB) may group a plurality of radio frequency (RF) beams into a beam group. The two dimensional codebook would increase the directivity of a RF beam of a beam group so as to increase the signal to interference plus noise ratio (SINR) of a user equipment (UE) under the coverage of the RF beam. However, the increase directivity of the RF beam would mean that the UE could slip through the coverage of RF beams and thus the resolution of the beam coverage would need to be increased. The resolution of a beam coverage could be increased or decreased by adjusting an oversampling rate of a beamforming weight. However, the increased oversampling rate could be mean that the signaling overhead required to select a precoder matrix would be increased. The oversampling rate would also affect the search space of a UE receiving. Specific details are elucidate in FIG. 1˜FIG. 7.
FIG. 1 illustrates a codebook for single layer CSI reporting by using antenna ports 0 to 3 or 15 to 18 as specified by 3GPP technical specification (TS) 36.213 which is incorporated by reference to define the concepts and definitions of the disclosure. The codebook is an example of a typical one dimensional codebook. As the codebook was modified to accommodate for multi-dimensional transmissions, the codebook has not only become more complex but also the overhead of precoder matrix indicator (PMI) has also been increased. A precoder from the codebook could be used by selecting a PMI.
FIG. 2 illustrates a two dimensional codebook as specified by 3GPP LTE release 13. For the disclosure, one of the parameters that could be of interests is N(x)O(x) 201 where N(x) represents codebookConfigN1 202 which indicates the number of antenna ports per polarization in dimension x as used for transmission of CSI reference signals. The format of codebookConfigN1 is stated on the bottom of FIG. 2 where the value n1 corresponds to 1 antenna port, n2 to 2 antenna ports and so on. Also O(x), represents codebookOverSamplingRateConfig-Ox 203 which indicates the spatial over-sampling rate in dimension x as used for transmission of CSI-reference signals, where the value n4 corresponds to 4 over-sampling rate, n8 corresponds to 8 over-sampling rate and so on. This codebookOverSamplingRateConfig-Ox is synonymous with “oversampling rate” in this disclosure. Further details with regard to the functionalities related to the oversampling rate are described in TS 36.213 and TS 36.331 which is incorporated by reference to define the concepts and definitions of the disclosure.
The two dimensional codebook as specified by 3GPP LTE release 13 for example has a dual-codebook structure. The precoding matrix in the two dimensional codebook is represented as W=W1W2 where the precoding matrix W1 is a long-term precoding matrix which is based on the long-term statistical properties of the channel and could be utilized by referred to a long-term precoding matrix indicator (PMI) and W2 is a short-term codebook which contains a short-term precoding matrix which could be utilized by referred to a short term PMI. The long-term precoding matrix
      W    ⁢                  ⁢    1    =            (                                                                  X                1                            ⊗                              X                2                                                          0                                                0                                                              X                1                            ⊗                              X                2                                                        )              2      ⁢              N        1            ⁢              N        2            ×      2      ⁢              L        1            ⁢              L        2            where X1X2 is the Kronecker Product (KP) operation. X1 is an N1×L1 matrix with L1 column vectors being a O1× oversampled DFT vector of length N1:
      v    1    =                    [                  1          ⁢                                          ⁢                      e                                          j                ⁢                                                                  ⁢                2                ⁢                π                ⁢                                                                  ⁢                l                                                              N                  1                                ⁢                                  O                  1                                                              ⁢                                          ⁢          …          ⁢                                          ⁢                      e                                          j                ⁢                                                                  ⁢                2                ⁢                                  π                  ⁡                                      (                                          N                                              1                        -                        1                                                              )                                                  ⁢                l                                                              N                  1                                ⁢                                  O                  1                                                                    ]            t        .  X2 is an N2×L2 matrix with L2 column vectors being a O2× oversampled DFT vector of length N2:
      v    2    =                    [                  1          ⁢                                          ⁢                      e                                          j                ⁢                                                                  ⁢                2                ⁢                π                ⁢                                                                  ⁢                l                                                              N                  2                                ⁢                                  O                  2                                                              ⁢                                          ⁢          …          ⁢                                          ⁢                      e                                          j                ⁢                                                                  ⁢                2                ⁢                                  π                  ⁡                                      (                                          N                                              2                        -                        1                                                              )                                                  ⁢                l                                                              N                  2                                ⁢                                  O                  2                                                                    ]            t        .  N1 and N2 are the number of antenna ports per polarization in 1st and 2nd dimension. For example, as shown in FIG. 3, one beam group 301 would contain 2 beams (L1L2=2). In addition, since an antenna setup thus creates quite pronounced channel properties, which are matched to a block diagonal structure of W1.
Short-term precoding matrix W2 is design for beam selection as well as co-phasing between the beams selected for two polarizations. More specifically, the short-term codebook W2 is designed with dynamic column selection for different antenna polarization and co-phasing φρ among selected beams. W2 can be represented as Rank 1:
            W      ⁢                          ⁢      2        =                  1                                            N              1                        ⁢                          N              2                                          ⁡              [                                                                                                                                e                n                                                                                                                                                            φ                p                                                                                                                                        e                n                                                    ]              ,where the co-phasing term is
      φ    p    =      e                  j        ⁢                                  ⁢        2        ⁢        π        ⁢                                  ⁢        p            4      as en denotes beam selection vector which is the n column of identity matrix IN1N2. An example selecting the 2nd beam 401 of a precoding matrix W1W2 is shown in FIG. 4.
An example of beam directions and related parameters is shown in FIG. 5. For FIG. 5, it is assumed that there are NdOd beams in the dth dimension. Referring to FIG. 6 which shows a two dimensional grid of beam map. In case of N1=2, for a first instance, for the first case 504 of O1=2, there could be a total of 4 beams. For a second instance, for the second case 505 of O1=4, there could be a total of 8 beams.
The parameters that are shown in FIG. 6 are defined as the following. The parameter i1d is first PMI or long term beam group index
      (                  e        .        g        .            ,                                    i            11                    =                                    0              ⁢                                                          ⁢              …              ⁢                                                          ⁢                                                                    N                    1                                    ⁢                                      O                    1                                                                    S                  1                                                      -            1                          ;                              i            12                    =                                    0              ⁢                                                          ⁢              …              ⁢                                                          ⁢                                                                    N                    2                                    ⁢                                      O                    2                                                                    S                  2                                                      -            1                                )    .For instance, the beam group index i11 indicates abeam group index in the 1st dimension (e.g. a first dimensional first PMI) and beam group index i12 indicates a beam group index in the 2nd dimension (e.g. a second dimensional first PMI). To put it plainly, the indices i11 and i12 determine a beam group which could be phrased as a long term PMI (e.g. first PMI). The index i2 selects a beam within the beam group which could be phrased as a short term PMI (e.g. second PMI). The parameter Pd is intra group beam spacing. The parameter sd is leading beam space between two adjacent groups or beam group spacing. The parameter Nd is numbers of port/TXRU per polarization. The parameter Od is oversampling rates, and the parameter Ld is number of beams in each beam group. The parameter L′d is beam group layout (e.g. L′1=4; L′2=2). The codebook configurations and their corresponding beam group parameters of the codebooks is shown in FIG. 7
As previously described, the legacy codebook design with fixed oversampling rate may not provide sufficient resolution to provide a full coverage since the larger antenna ports may implicate smaller beam coverage caused by highly directive antenna beams. However, an increased oversampling rate may implicate greater burden for CSI reporting because of the increased codebook size. The inefficient use of oversampling rate may also adversely impact the computational efficiency. Therefore, a dynamic beamforming method and related apparatuses that use the same method could enhance the above stated shortcomings.